Challenges are presented in attempting to manufacture a toroidal-shaped pneumatic radial tire consistently from tire to tire using sheet or strip materials. For example, a typical pneumatic radial tire includes a pair of axially spaced apart and circumferentially inextensible beads. A carcass ply extends between the beads and is attached to a respective bead at axially opposite end portions of the carcass ply. The carcass ply includes a plurality of parallel reinforcing members that extend between the beads as well. The carcass ply is formed into a toroidal shape and typically has a belt package located radially outward of the carcass ply in a crown portion of the tire. Tread rubber and sidewall rubber are applied over the belt package and carcass ply, respectively.
Once the tire is assembled and cured, the tire may be tested for a uniformity characteristic. “Uniformity” is defined herein as what a “perfect” or “ideal” tire would yield for certain measured characteristics when tested during rotation. “Uniformity characteristic” is defined herein as a deviation in a certain characteristic from what the perfect tire would yield during testing for that same characteristic.
Testing a tire for a uniformity characteristic typically begins with mounting the tire in an inflated condition on a test spindle of a uniformity tester. A test wheel is moved into engagement with the tire to radially deflect a portion of the tire a predetermined amount. The position of the axis of rotation of the test wheel relative to the axis of rotation of the tire is then fixed by a locking mechanism. The test wheel is rotated to cause rotation of the tire. Sensors associated with the test wheel sense radial and lateral forces transmitted by the tire to the test wheel during rotation of the tire.
One uniformity characteristic test which is generally performed on the tire is a test for radial force variation. Radial force variation is typically expressed as a variation in the force against the test wheel that is sensed during rotation of the tire. Radial force variation can be represented by a combination of first harmonic radial force variation through an Nth harmonic radial force variation or as a composite radial force variation. The Nth harmonic is the last harmonic in a Fourier Series analysis of the composite radial force variation which is deemed acceptable to accurately define the radial force variation. Vehicle ride is generally most affected by the first harmonic radial force variation of the tire. The first harmonic radial force variation is often associated with “radial runout” of the tire. Radial runout is defined as a difference in the radius from the axis of rotation to the outer periphery of the tire tread around the tire.
Another uniformity characteristic test which may be performed on the tire is a test for conicity, which is defined as the tendency of a rotating tire to generate a lateral force regardless of the direction of rotation of the tire. Conicity is expressed in terms of average lateral force generated during rotation in both directions of the tire against the load.
Such uniformity characteristics may be attributed to the manufacture of a tire from the sheet and/or strip material. The uniformity characteristics can simplistically be viewed as a deviation from perfect roundness of the outer circumference of the tire, as deviation from spindle load transmitted by a perfect tire during rotation (radial force variation) or as deviation from straight tracking during rotation (conicity).
If the uniformity characteristic of the tire has a magnitude which is less than a predetermined relatively low minimum magnitude and which is deemed not to be detrimental to a vehicle ride or produce undesirable vibrations in the vehicle, the tire may be shipped to a customer. If the uniformity characteristic magnitude is greater than a predetermined maximum threshold magnitude, the tire is scrapped. If the uniformity characteristic magnitude is between the relatively low minimum magnitude and the maximum threshold magnitude, the tire may be suitable for correction.
U.S. Pat. No. 5,616,859, owned by inventors' assignee and incorporated herein by reference, describes an advantageous method and apparatus for tire uniformity correction without grinding. For example, a signal is generated to indicate the magnitude and location of a uniformity characteristic of a tire. At least a portion of one carcass reinforcing member of the tire is permanently deformed at a location determined from the signal so as to correct the uniformity characteristic.
Certain difficulties are encountered when attempting permanent deformation of a carcass member in order to correct a uniformity characteristic in a low profile tire or a tire having a projection along the sidewall such as e.g., a rim protector. As used herein, “low aspect ratio tire” or “low profile tire” includes a tire having a relatively short sidewall height. For low profile tires, attempts to permanently deform only one portion of the carcass reinforcing member are frustrated by several factors including e.g., high carcass strength and short cord length. For tires that have a sidewall feature such as a rim protector, the projection from the sidewall can interfere with the proper constraint of the sidewall during deformation. In either case, certain features of the tire may lead to deformations at unintended locations within the architecture of the tire. Accordingly, a need exists for means by which the correction of uniformity characteristics can be achieved in a low profile tire or a tire having certain projections along its sidewall without introducing undesired deformations.